1. Field of the Invention
The present invention relates to the fabrication of integrated circuits (IC's). More particularly, the invention provides a technique, including a method and apparatus, for forming a gate stack structure having an improved gate electrode connection structure formed by chemical vapor deposition (CVD) of tungsten (W) and plasma enhanced chemical vapor deposition (PECVD) of tungsten nitride (W.sub.x) films.
2. Description of the Background Art
Modern integrated IC's contain large numbers of transistors. These transistors are generally field effect transistors that contain a source region and a drain region with a gate electrode located in between the source and drain regions.
A typical gate structure contains a thin polysilicon electrode that lies on top of a thin layer of gate oxide such as silicon dioxide (SiO.sub.2). The gate electrode and gate oxide are formed between semiconducting source and drain regions, that define an underlying well of p-type or n-type silicon. The source and drain regions are doped opposite to the well to define the gate location, a layer of insulating material such as silicon oxide (SiO.sub.x) or silicon nitride (SiN.sub.x) is deposited on top of the source and drain regions and an aperture or via is formed in the insulating material between the source and drain regions. The gate structure within the via contains a thin oxide layer, a polysilicon layer and a metal plug. The metal plug is formed by vapor depositing a metal such as tungsten on top of the polysilicon gate electrode. To complete the connection, the silicon then is caused to diffuse into the tungsten during a thermal annealing process forming a layer of relatively uniform tungsten silicide (WSi.sub.x) as the connection to the gate electrode. Without annealing, the silicon will ultimately diffuse into the tungsten forming a non-uniform layer of tungsten silicide.
A gate electrode having an electrical connection made of pure tungsten would be more desirable than a tungsten silicide electrode since tungsten has a lower resistivity than tungsten silicide. Unfortunately, silicon diffuses into the tungsten forming tungsten silicide. The diffusion can be prevented by depositing a layer of tungsten nitride (W.sub.x N) as a diffusion barrier. W.sub.x N is a good conductor as well as an excellent diffusion barrier material. Such a barrier layer is formed by reducing tungsten hexafluoride (WF.sub.6) with ammonia (NH.sub.3) in a chemical vapor deposition (CVD) process.
Unfortunately, the above described process results in the formation of contaminant particles in the form of solid byproducts. These byproducts include ammonia adducts of tungsten hexafluoride ((NH.sub.3).sub.4 WF.sub.6), ammonium fluoride (NH.sub.4 F) and other ammonium complexes. Many of these particles become attached to the deposition chamber's interior. During temperature fluctuations within the chamber, the deposits flake off the walls and contaminate the wafer. Further, the tungsten nitride that is deposited using the above described process has a polycrystalline structure in which there are many grain boundaries. As a result, the diffusion barrier properties of the tungsten nitride are compromised. In addition, tungsten nitride films deposited by the traditional method tend not to adhere very well to the substrate upon which they are deposited.
Therefore, a need exists for a gate structure having a low resistivity tungsten gate electrode connection with a compatible diffusion barrier to prevent diffusion of silicon into the tungsten and a concomitant method and apparatus for manufacturing same.